The Relay Testing Handbook
Three-phase systems can be either clockwise (A-B-C) or counterclockwise (A-C-B) rotation.
The phase rotation of the system can be determined by the rotor direction of rotation, or
the position of the windings. To correctly determine phase rotation, imagine that the sine
waves are constantly moving from right to left and pick a vertical line as a reference. Notice
which order the positive peaks pass through your reference. The “A” wave will cross first
followed by “B “, and then “C” which is A-B-C rotation. We always write A-B-C and A-C-B as
a standard even though B-C-A and C-A-B are technically possible in an A-B-C system: and
C-B-A and B-A-C are technically correct for an A-C-B system. Use the examples in Figure
1-3 to help understand phase rotation.
A
A
C
B
+
B
VOLTS
A
A
B
C
+
N
0
5
S
270
5
31
22
VOLTS
45
5
13
90
180
B
C
-
COUNTER-CLOCKWISE ROTATION EQUALS A-B-C ROTATION
A
A
C
B
+
N
0
5
S
270
5
31
22
VOLTS
45
5
13
90
180
C
B
-
CHANGING ANY TWO POLES EQUALS A-C-B ROTATION
A
C
B
A
+
45
N
5
13
90
5
S
270
5
31
22
VOLTS
0
180
B
C
-
CLOCKWISE ROTATION EQUALS A-C-B ROTATION
Figure 1-3: Phase Sequence Examples
4